In general, foamed polymers are materials which have a cellular structure, which means gas filled cells are embedded in the polymer matrix. These cells can be isolated from each other, (closed cell foam), or they can have openings to each other, (open cell foam). Open and closed cell foams have mostly common properties, but they have also some different properties. The main difference is that the volume of the cells in open cell foams is accessible from the outside. Therefore, gaseous or liquid materials can penetrate the foam and can be stored and released again without damaging the foam itself. The open cell structure also allows this type of foam to be used as a filter, which prevents solid particles larger than the cell from passing through the foam.
Polymer foams have certain typical properties compared with the solid unfoamed polymer. They have a lower density, lower heat and electrical conductivity, and lower strength. The lower strength can be explained by the reduced amount of polymer per cross section.
There are different processes which can create polymer foams. The most important ones are Reaction Injection Molding (RIM), injection molding and foam extrusion. In Reaction Injection Molding the polymer is foamed by carbon dioxide which is released as a byproduct of the chemical reaction between the two monomers. In injection molding and foam extrusion, a blowing agent has to be added to the polymer. There are two types of blowing agents, chemical and physical blowing agents. Chemical blowing agents are solids which decompose at elevated temperatures. The decomposition products are normally a mixture of gases and solid byproducts. Chemical blowing agents are normally blended with the polymer pellets in the form of a dry powder. This makes their use very convenient, because no change of the conventional extrusion or injection molding equipment is necessary.
Regardless of whether chemical or physical blowing agents are used, additional nucleation agents are needed to enhance nucleation of the gas bubbles in conventional foaming systems. The nucleating agents are normally solids with a very small diameter and are blended into the polymer pellets. To achieve a very uniform and homogeneous cell structure it is necessary to distribute the nucleation agents very uniformly throughout the polymer melt. The smallest cell size of polymer foams processed with these conventional methods is in the range of 100 .mu.m. The corresponding cell density, which is defined as cells per cubic centimeter of the unfoamed material, is in the range of 10.sup.3 to 10.sup.6 cells per cubic centimeter.
Microcellular polymers are polymer foams with much smaller cell sizes than those of conventional polymer foams. They are generally defined as foams with cell sizes under 100 .mu.m and cell densities on the order of 10.sup.8 and larger. Recently developed supermicrocellular polymers have even smaller cells and higher cell densities. Supermicrocellular polymers are defined by cell sizes smaller than 1 .mu.m. The cell densities of these polymers are normally larger than 10.sup.12 cells per cubic centimeter. The smallest cell size which could be achieved so far is 0.1 .mu.m which corresponds to a cell density of about 10.sup.15 cells per cubic centimeter.
Microcellular polymers have certain advantages over conventional foamed polymers. First, their cells are so small that they can not be seen by the naked eye. Therefore, the physical appearance equals that of the solid polymer. However, the microstructure lets them appear opaque and, if no color pigments are used, they appear white. Furthermore, the mechanical properties change significantly. They are much tougher, have increased specific mechanical strength and have a much longer fatigue life than the solid polymer (Collias and Baid, 1992: Seeler and Kumar, 1992, 1993). In addition, their cell structure and cell size distribution is much more uniform than that of the conventional foamed polymers, which results in generally much better properties than those of conventional polymer foams. The increase of the fatigue life of microcellular polymers can be explained by the huge amount of cells per cubic centimeter. The cells act as crack arresting sites. Therefore, microcracks are stopped at once and the chance that they result in fatigue fractures is reduced significantly.
Applications of microcellular and supermicrocellular polymers are many. Not only can they can replace mostly all of the conventional foams due to their better properties, they also fill niches for new applications which have not existed up until now. Another application is the substitution of the solid polymer with microcellular polymers so as to increase the fatigue life of the product, without significantly sacrificing the mechanical strength. This is possible due to very low cell expansion with minimal density increase. The large number of cells act as crack arresting sites which increase the fatigue life; nevertheless, the mechanical strength is still close to that of the original polymer because of the low volume expansion.
Another possible application is the development of transparent supermicrocellular foams. A requirement for this material is that the cells are much smaller than the wavelength of light, which is in the order of 0.05 .mu.m.
A huge advantage of microcellular polymers is that their processing does not require the use of fluorochlorocarbons, which is very important from the environmental point of view. Environmentally safe gases like carbon dioxide or nitrogen are used for their processing.
All processing techniques for microcellular polymers have in common one fundamental process, which basically consists of three steps; (1) formation of a single-phase polymer/gas solution, (2) cell nucleation and (3) cell growth.
The first step in creating a microcellular polymer foam is the formation of a single-phase solution of polymer and gas. Two physical phenomena are involved in this process, the diffusion of the gas into the polymer and the attainment of thermodynamic equilibrium between the polymer and the gas. Under equilibrium conditions, a certain amount of gas is dissolved in the polymer. The solubility of a gas in a polymer is a strong function of the pressure and the temperature. It increases with pressure and decreases with temperature.
At the moment when the polymer is suddenly set under external pressure by a gas, the whole system of gas and polymer is in a thermodynamically unstable state. The gas starts diffusing into the polymer to increase the entropy of the system and to finally reach a new stable thermodynamic equilibrium state. The driving force for diffusion is therefore the Gibbs free energy. Solubility of gas in polymer is a function of pressure and temperature. The diffusivity D is a function of the temperature and increases with increasing temperature. The relation between diffusivity and temperature can be approximated by an Arrhenius equation: EQU D=D.sub.0 e.sup.-.DELTA.G/kT
where .DELTA.G is the activation energy, k the Bolzmann's constant, and T the absolute temperature (Newitt and Weale, 1948: Duml and Griskey, 1966 and 1969; van Krevelen, 1976; Koros and Paul, 1980). This equation is valid only if the change in gas concentration in the gas/polymer system does not effect the diffusivity. In the case of microcellular polymer processing, the influence of the concentration change is not negligible, because the amount of gas dissolved in the polymer at equilibrium is on the order of six to ten percent by weight of the polymer (Park MIT Thesis, 1993).
The most essential step in creating microcellular polymers is the nucleation of the cells. Unlike conventional foam processing, no nucleation agents need to be used for the cell nucleation in microcellular polymer processing. Classical nucleation theories distinguish between homogeneous and heterogeneous nucleation. Throughout a homogeneous matrix the activation energy required for nucleation is totally uniform. Therefore, nucleation itself occurs totally uniformly throughout this matrix. Heterogeneous nucleation occurs at interfaces of two or more different materials or at the interfaces of the different microstructures of one material, e.g., amorphous and crystalline phase in semi-crystalline polymers. At these interfaces the interfacial energy is high and as a result the activation energy required for nucleation is low.
The driving force for nucleation in microcellular polymer processing is the supersaturation of the polymer due to the rate of changes in pressure or temperature. As demonstrated by Park (1993), Baldwin (MIT Thesis,1994), and others in extrusion processing, microcellular nucleation rates are created when solubility levels change at rates far above that of conventional extrusion foaming. These solubility rate changes are caused by thermodynamic instabilities of either pressure drop (Park) or temperature rise (Baldwin).
The polymer/gas single-phase solution was at a stable equilibrium state at high pressure. The high rates of drop in pressure or the rates of increase in temperature change the solubility of the gas in the polymer very rapidly which is a very large driving force. The difference in the activation energy level for homogeneous and heterogeneous nucleation is small in comparison to the energy available for nucleation. It exceeds the energy required for heterogeneous nucleation and also allows for homogeneous nucleation simultaneously. The higher the degree of supersaturation, the higher is the degree of homogeneous nucleation, independent of the number of heterogeneous nucleation sites. For that reason no nucleation agents are needed. For a more detailed discussion of classical and newer nucleation models for microcellular polymer processing, refer to Baldwin (1994), Park (1993).
Nucleation alone however, does not create a microcellular structure. The third and most difficult to control step in microcellular polymer processing is cell growth. Right after the nucleation of a cell the pressure in the nucleus is equal to the saturation pressure (Suh, 1995). Therefore, the bubble tries to expand. The expansion is constrained by the surface tension of the bubble. The internal pressure of the bubble and the surface tension of the bubble determine the size of the cell at equilibrium condition. The cell grows only if the polymer matrix is soft enough to allow expansion. During cell growth, gas diffuses out of the polymer matrix and into the bubble. Since the amount of cells nucleated in microcellular polymer processing is very large, the amount of gas has to be distributed among all the nucleation sites. Therefore, the amount of gas available for each cell is very small, which results in very small cell sizes.
The basic process for microcellular polymers can be applied to several processing techniques, including batch processing and extrusion processing. During batch processing a polymer sample is placed in a pressure chamber and the chamber is pressurized with a gas. The gas saturates the sample as it diffuses into it. The time required to saturate the sample depends on the thickness of the sample, the gas pressure, and the temperature, and is in the order of minutes to days. Once the sample is saturated completely, the gas is released quickly, to cause nucleation. The conditions for cell growth depend strongly on the type of polymer and the polymer itself. Some polymers foam at room temperature while other polymers need to be heated to much higher temperatures to enable cell growth. In general, foaming temperatures lie in the range of the glass transition temperature (T.sub.g) of the polymer/gas solution using amorphous polymers. Note that the glass transition temperature of the single-phase solution of gas and polymer is significantly lower than the glass transition temperature of the neat polymer. This is due to the softening effect of the gas in the polymer matrix. (Cha thesis).
This effect (drop in Tg) and others, characteristic of high saturation levels of CO.sub.2 or other supercritical blowing agents, upon processing conditions has been discussed in Park (1993).
Semicrystalline polymers, however, require foaming temperatures around the melting point (T.sub.m) of the crystalline areas in the polymer matrix. If the polymer matrix in these areas is not soft enough only the amorphous regions are foamed, resulting in fewer and therefore bigger bubbles.
The basic microcellular polymer process can also be incorporated in an extrusion process. The extrusion equipment necessary consists basically of four physical units. There is the extruder which plasticates conventional polymer pellets. The gas is injected directly in the polymer melt flow of the extruder by a gas metering system. Mixing and homogenizing sections of the extruder screw create the single-phase solution of the polymer and the gas. Additionally, the extruder creates the pressure required for a high gas solubility, the key issue for a high nucleation rate. Nucleation is achieved by a nucleation device at the end of the extruder. The original nucleation device used by Park (1993) was a single-orifice nozzle which utilized a rapid pressure drop for cell nucleation (see FIG. 1). The pressure drop was achieved by the viscous flow of the polymer/gas solution through the nozzle. One limitation of the nozzle pressure drop device was very low flow rates. Later, near net shape nucleation was investigated by Baldwin (1994), which showed that only limited product qualities could be achieved. Cell sizes with nozzle and near net shape nucleation were in the range of 10 .mu.m. The bubbles were sheared in extrusion direction which resulted in nonuniform product properties.